Rechargeable metal halide battery

ABSTRACT

Electrode structures for a rechargeable metal halide battery include: a cathodic electrode comprising a halogen-inert electroconductive layer and bonded to one of the major surfaces thereof, a halogen-entrapment structure comprising a halogenadsorbent layer and a surface layer comprising porous, electrically non-conductive halogen-inert, electrolyte-inert, and halogen non-adsorbent particles, having an average largest dimension less than about 10 mils and a halogen-inert bonding agent bonding the particles together into an integral nonconductive mass which essentially retains the porosity of the particles; and an anodic electrode having a coating of such electrically non-conductive particles on the electroplating surface of the electrode.

United States Patent Zito, Jr. 1 March 6, 1973' 1 RECHARGEABLE METALHALIDE Primary Examiner-Helen McCarthy BATTERY Assistant Examiner-41. A.Feeley Attorney-E. H. Kent [75] Inventor: Ralph Zito, Jr., Westford,Mass.

[57] ABSTRACT [73] Assignee: The Zito Company, Inc., Derry,

Electrode structures for a rechargeable metal halide 2. battery include:a cathodic electrode comprising a [22] Filed: Feb. 3, 1971 halogen-inertelectroconductive layer and bonded to one of the major surfaces thereof,a halogen-entrap- [21] Appl' 112254 ment structure comprising ahalogen-adsorbent layer and a surface layer comprising porous,electrically [52] 0.5. CI ..136/6, 136/86 A, 136/120 FC n nn l g nelectrolyte-inert, and [51] 1111.01. ..l-l01m35/00 halogen non-adsorbentparticles, having an average [58] Field of Search ..l36/6, 30, 86 D, 86A, 120, largest dimension less than about 10 mils and a 13 122 123 124177 179 halogen-inert bonding agent bonding the particles together intoan integral non-conductive mass which 5 R f cit d essentially retainsthe porosity of the particles; and an anodic electrode having a coatingof such electrically UNITED STATES PATENTS non-conductive particles onthe electroplating surface 3,285,781 11/1966 2110 ..l36/6 electmde3,328,202 6/1967 Riffe... ..l36/30 15 C i 14 Drawing Figures 3,382,1025/1968 Zrto ..l36/30 3,425,875 2/1969 Adlhart et a1. ....136/1223,432,355 3/1969 Niedrach et a1. ..l36/l20 RECHARGEABLE METAL HALIDEBATTERY This invention relates to metal halide storage batteries andalso to similar devices requiring halogen storage systems.

In a rechargeable metal halide storage battery, a metal halide salt,dissolved in a suitable electrolyte, is electrolyzed during chargingproviding free metal at the anode and molecular halogen at the cathode.If the battery is to retain substantial charge after the chargingcurrent is discontinued, the metal and halogen must be storedsubstantially out of chemical contact with one another, and yet still beaccessible on demand when power is to be drawn from the battery. Thismust be accomplished in a compact battery construction, with a smallelectrolyte volume, if the battery is to be an economical energy source.Moreover, to have an adequate useful life, the storage system providedmust function reliably and without deterioration over repeated chargingand discharging cycles.

The object of this invention is to provide compact, reliable andeconomical metal halide energy sources, such as rechargeable storagebatteries, having improved charge retention capacity.

Another object is to provide halogen storage systems for storing halogenin molecular form for substantial periods of time in a medium renderingthe halogen substantially instantaneously accessible on demand forelectrochemical reaction.

A further object is to provide improved electroplating surfaces forreversibly storing electroplated metals in a medium rendering theelectroplated metal substantially instantaneously accessible on demandfor electrochemical reaction.

A particular object is to provide improved cathode and anode structuresfor metal halide batteries which are light-weight, simple and economicalto produce in mass quantities, and which interact with halogen/halideand metal/metal ions, respectively, in a consistent, reproduceablemanner during repeated charging and discharging operations.

Another object is to provide improved rechargeable zinc bromide storagebatteries.

The invention features electrode structure for a rechargeable metalhalide battery in which a salt of an electroplatable metal and a halogenselected from the class consisting of chlorine, bromine and iodine iselectrolyzed from solution in a liquid electrolyte medium during thecharging cycle and reformed during the discharging cycle. In one aspect,the invention features electrode structure having improved cathodecharacteristics comprising a halogen-inert electroconductive layer,substantially impermeable to the electrolyte, and substantiallyimpermeable to halogen between discharging cycles, which layer hasbonded to it along one of its major surfaces, a halogen-entrapmentstructure, which is permeable to electrolyte, inert to halogen, andcomprises a halogen-adsorbent layer adjacent the electroconductivelayer, and, at least along and bonded to the major surface of theadsorbent layer (opposite that bonded to the electroconductive layer) asurface layer comprising porous, electrically non-conductivehalogen-inert, electrolyte-inert, and halogen non-adsorbent particles,having an average largest dimension less than about mils and ahalogen-inert bonding agent bonding the particles together into anintegral non-conductive mass which essentially retains the porosity ofthe particles. When the battery is charged, this surface layer retardshalogen diffusion out of the adsorbent layer and into the electrolyte,where it could otherwise migrate to and attack the metal elec troplatedat the anode surface of the battery. The charge storage capacity of thebattery is substantially increased over batteries utilizing adsorbentlayers alone (of substantially the same thickness). Yet, this addedsurface layer, being non-reactant with and non-adsorbent of halogen,will not adversely affect the rate at which halogen can recombine withmetal in the battery during discharge, thus permitting selected highdischarge rates. The halogen will not form at this surface layer in thecharge cycle, since the layer is electrically non-conductive, but willform beneath the layer, and hence will not tend to be exposed to theelectroplated anode surfaces when the battery is left at full charge.This surface layer is also preferably thin (generally about one-third orless of the total thickness of the underlying adsorbent layer), hencepermitting close spacing of electrodes, minimal electrolyte volume, andresultant light-weight and compact battery construction.

Where the halogen is bromine, a preferred adsorbent layer comprisesbromine-adsorbent activated carbon particles (preferably comprisingpercent or more of the total weight of the layer) bonded together intoan integral adsorbent mass by a bromine-inert bonding agent, theadsorbent layer having a bromine adsorptivity of at least 0.5 gms ofbromine per gram of adsorbent layer. In a preferred entrapment devicehaving such an adsorbent layer, the surface layer bonded thereto hasedge portions of greater thickness than its interior portion, and theedge portions of the adsorbent layer are of correspondingly reducedthicknesses, so that the exposed cathode surface of the surface layermay be flat.

This edge construction assures that the major adsorbent surfaces will befully covered by the surface layer, further improving the chargeretention capacity of the battery. In such constructions, a preferredthickness for the thinner interior portion of the surface layer is about10 to 30 mils.

In another aspect, the invention features electrode structure, havingimproved anode characteristics, comprising a halogen-inertelectroconductive member substantially impermeable to said liquidmedium, substantially impermeable and inert to halogen at least betweendischarging cycles, and which has an exposed surface, adapted to bearranged as a metal electroplating surface, to which is firmly bonded acoating of the aforesaid non-conductive particles of a thicknesssufficient to substantially cover the exposed surface. It has been foundthat such an electroplating surface results in a more consistent opencircuit voltage at successive charging cycles, indicating a smoother,more uniform, and stronger electroplate deposit. The discharge cycle isalso found to be steady, smooth and predictable. It is believed that theelectrical non-conductivity of the coating causes metal to commenceplating at the interface of the electroconductive layer and the coating,and hence well within the coating itself. Subsequent metal layer growthbeing within and through this coating, including within the porousinteriors of the particles, the electroplated metal layer has strengthand integrity. Further, since substantial portions of this electroplateare somewhat isolated from whatever free halogen may be in theelectrolyte, corrosion or dissolution of the electroplate during storageof the battery in a charged condition is considerably impeded. Theuniformity of the electroplate deposit over the entire electroplatingsurface further discourages the growth of metal dendrites through theelectrolyte which, upon reaching electrolytic cathode surfaces, cancause discharge in or even short-circuit of the battery. In

preferred embodiments, this coating is about to 20 mils thick.

In the preferred anodic and cathodic structures, the electroconductivelayer or member comprises highly electro-conductive carbon particles(e.g., graphite) bonded together into an integral electroconductive massby a bonding agent substantially inert to bromine and to theelectrolyte, the mass being preferably 25 percent to 75 percent carbonparticles by weight.

Other objects, features and advantages will appear to one skilled in theart from the following description of a preferred embodiment of theinvention taken together with the attached drawings thereof, in which: 7

FIG. 1 is a plan view of a metal halide battery embodying the presentinvention, with the forward side of the battery housing partially brokenaway;

FIG. 2 is a sectional view of a terminal cathode constructed inaccordance with the present invention;

FIG. 3 is a sectional view of a composite or intermediate electrodeconstructed in accordance with the present invention;

FIG. 4 is a sectional view of a terminal anode constructed in accordancewith the present invention (or used in the battery of FIG. 1

FIG. 5 is a plan view, partially broken away, of the cathode of FIG. 2;

FIG. 6 is a plan view, partially broken away, of the terminal anode ofFIG. 4;

FIGS. 70, 8a, 9a and 100 are plan views of electrode structures usefulin forming the electrode shown in FIG. 3; and

FIGS. 7b, 8b, 9b and 10b are sectional views of the electrode structuresof FIGS. 7a, 8a, 9a and 100, respectively.

FIG. 1 shows a zinc bromide battery 10 encased in a polyethylene housing12, of which the forward side has been partially broken away to exposethe interior of the battery. The battery has a terminal cathode l4electrically connected to a cathode terminal screw which extends throughthe housing 12, a terminal anode 18 at its other end which iselectrically connected to an anode terminal screw 20 which also extendsthrough the housing 12, and a plurality (seven) of intermediate orcomposite electrodes 22, all arranged generally in parallel and spacedapart from one another. A liquid electrolyte medium 23 consists of anaqueous solution which is 0.5 to 7 molar zinc bromide, 0.01 molaraluminum chloride, and 0.005 molar aluminum potassium aluminum sulfate(the aluminum salts being brightener additives). This battery is about14 volts, and has a capacity of over 20 amp/hour at full charge.

Referring to FIGS. 2 and 5, the terminal cathode 14 has a copper screen26, the major portion of which is sandwiched between two bromine-inertand bromineimpermeable electroconductive layers 28, 29, each of which isformed of a 50-50 by weight mixture of electroconductive graphiteparticles bonded together by poly (vinylidene fluoride) (Kynar)particles, the graphite and bonding agent being bonded together underheat and pressure to form an integral electroconductive layer. Eachlayer 28, 29 is capable of conducting electricity across its thicknessfrom its exposed face through copper screen 26 and has a thickness ofabout 25 mils. The layers 28, 29 are bonded to one together through theopenings in copper screen 26.

Firmly secured to the exposed face 30 of layer 29, which has an exposedsurface area of about 120 (10 X 12 inches) square inches is a bromineentrapment structure 32, which consists of an interior bromine adsorbentlayer 34 and a surface layer 38 firmly secured to the front exposed face36 of adsorbent layer 34. Adsorbent layer 34 is formed of at least aboutpercent by weight of bromine adsorbent activated carbon particles, andthe remainder of a polyethylene bonding agent effective to bond thecarbon particles into an integral adsorbent mass. The layer 34 has athickness on the order of about mils over most of its area, and asomewhat reduced thickness at its edges 37.

Surface layer 38 is about 30 mils thick over most of its area, and ofsomewhat greater thickness at its edges 42, and is formed of at leastabout 90 percent by weight of electrically non-conductive particlesbonded together into an integral non-conductive mass by a polyethylenebonding agent. The particles are porous, inert to halogen andelectrolyte, and do not adsorb halogen. The edges 37 of adsorbent layer34 are rounded around the entire periphery of the layer, and the edges42 of the surface layer 38 are sized so that the bromine entrapmentdevice 32 will have a rectangular cross section in all threeperpendicular planes, presenting a flat extended cathode surface 44 tothe electrolyte.

Each composite electrode 22, shown in FIG. 3, has a singleelectroconductive layer 46, identical to either of layers 28 and 29 ofterminal cathode l4, and approximately 25 mils thick. A thin coating 50(on the order of 10 mils) of the above-described electricallynon-conductive particles is bonded to the exposed anode surface 48 oflayer 46. These particles are bonded directly to the poly (vinylidenefluoride) bonding agent of electroconductive layer 46. Secured to theopposite surface 52 of electroconductive layer 46 is a bromineentrapment structure 54 identical to the bromine entrapment structure 32of terminal cathode 14.

Referring to FIGS. 4 and 6, terminal anode 18 has a copper screen 56,identical to copper screen 26 of terminal cathode 14, the major portionof which is sandwiched between two bromine inert andbromine impermeableelectroconductive layers 58, 59,. which are identical to theelectroconductive layers 28, 29 of terminal cathode 14 and areidentically bonded to one another through the openings of screen 56. Theexposed anode face 62 of electroconductive layer 58 has a thin coating64 of the above-described electrically non-conductive particlesidentical to the coating 50 on the anode face 48 of composite electrode22. Exposed portions of the electroconductive layers, and bromine'adsorbent layers are coated with a thin coating 66 of a bromine-inertand gas-impermeable sealer, such as the silicon rubber sealer availablefrom'General Electric Company under the trade name RTV.

Although the battery shown in FIG. 1 is electrically tapped as shownonly at the terminal electrodes 14 and 16, it will be understood thatlesser voltages may be tapped by providing intermediate terminal-typeelectrodes. Such electrodes would be identical to terminal cathode 14,except for the presence of a thin coating of electrically non-conductiveparticles on the exposed major surface of the electroconductive layer28, identical to the coatings 50, 64 on composite electrode 22 andterminal anode 18, respectively.

The battery housing 12 has in its cover 80 and opening 81, sized toreceive a generally cylindrical gas escape cap 82 which seals theinterior of the battery. As shown in FIG. 1, cap 82 is formed of agas-permeable (but liquid and solid-impermeable) and bromine-inertmaterial, such as polyethylene. The cap is at least partially filledwith a material which will remove bromine from gases coming from thebattery through the interior cap wall, but not other gases, such asoxygen and hydrogen. Finely-divided zinc filings is a suitable suchmaterial, as is particulate activated carbon. The vent cap 82 is sizedto fit in a tight seal into opening 81 and has a projecting flange whichwith a sealing gasket, provides a gas tight seal between the cap 82 andthe cover 80. Since the gas escape cap is gas-permeable, but at leastits interior wall is liquid-impermeable, hydrogen and oxygen gasesevolved in small quantities during operation of the battery (but notbromine or liquid) will escape through the exterior cap wall to theatmosphere.

The bottom wall 91 of housing 12 has a number of upstanding ribs 92,defining therebetween grooves 94. A spacer 95 has the electroductivelayers protruding therethrough, and sealed therein by a bromine-inert,and bromineand liquid-impermeable epoxy resin 96. The spacer 95 has anelongated opening 97 (about 1 inch wide) for filling the battery.Screens 26, 56 and screws 15, 20 are also potted, as shown, in resin 96.

When the electrodes are secured in place, the battery is filled withelectrolyte solution by vacuum impregnation, so that thebromine-adsorbent device becomes saturated with electrolyte all the wayin to the electroconductive (graphite-fluorocarbon) layers. The batteryis charged in the usual manner across the terminal screws 15, 20,electrolyzing the zinc bromide salt to form moleuclar bromine which isadsorbed by the activated carbon of the cathode and metallic zinc, whichis electroplated onto the anode surfaces. During the discharge cycle,the opposite electrochemical reaction takes place, the moleuclar brominereturning to bromide ion and the zinc plating dissolving to zinc ions.

Although the electrochemical system illustrated is the zinc bromidesystem, the illustrated electrode structures may be suitable for usewith other metal halide systems, wherethe halogen is chlorine or iodine,or the metal is other than zinc. Among the other metals which arereversibly electroplatable, and form water-soluble metal halide saltsare nickel, cadmium, tin, lead and copper. In a non-aqueous electrolytemedium, such as might be utilized for chlorine, the list of metals mightalso include sodium, potassium and lithium. Of these systems, the zincbromide system has the advantages of providing a reasonably highpotential (1.83 volts), using a very soluble salt which provides a lowresistivity electrolyte, and having a calculated free energy per poundof about 200 watt/hours. The molarity of a zinc bromide electrolyteduring charging and discharging is preferably between about 0.5 and 7.The electrolyte also may contain a brightener to improve zincelectroplating.

Among preferred particles for forming the cathode surface layers andanode coatings are those commonly used as filter materials or filteraids such as diatomaceous earth, molecular sieves, zeolites and thelike. These particles are characterized by a high water absorptivity,typically more than two times their weight, and an electricalresistivity on the order of l0 ohm-cm or even greater.

A particularly preferred material is a flux-calcined diatomaceous earthavailable from Johns-Manville Company under the trade name Celite 560.

The pore size of the particles must be large enough to permit diffusionof electrolyte therethrough but small enough to at least substantiallyretard diffusion of molecular bromine. Preferred particle layers arecomposed of particles, the pores of which have an average effectivediameter less than about one micron, and

preferably lower, down to molecular size.

The size of the non-conductive particles is chosen to provide surfacelayers at least several particles thick. It is preferred thatsubstantially all of the particles have a largest dimension less than 10microns. Particles which pass a mesh screen are preferred (the aforesaidCelite 560 particles so passing, but being 60 percent retained on a meshscreen). Particles in the micron range'and below are also useful, solong as the particles are not so small as to be occluded by the bondingagent used. It is preferred that the absorptivity of the surface layerbe essentially that of the particles themselves, with the amount ofbonding agent being that barely sufficient to provide an integral layer.Preferably, the surface layer is at least about 90 percent by weight ofthe particles, and no more than about 10 percent by weight of bondingagent.

The bonding agent in the surface layer must be adherable to that in theadsorbent layer, and, preferably, is identical thereto. Useful bondingagents include polyfluorocarbons, such as polytetrafluoroethyleneTeflon, available from E. I. de Pont de Nemours & Co.), poly (vinylidenefluoride) (Kynar", available from Penwalt Co.),polymonochlorotrifluoroethylene (CTFE, available from Allied ChemicalCo.), and FEP, a fluorinated polyethylene available from the same duPont; poly (vinyl chloride) homopolymers (plasticized or unplasticized)e.g., Geon 222, available from B. F. Goodrich Co.); poly (vinylidenechloride) homopolymers and copolymers (50 percent or greater vinylidenechloride) such as acrylonitrile and vinyl chloride copolymers (availablegenerally under the trade name Saran" from Dow Chemical Co.);poiymethacrylates such as poly (methyl methacrylates) (Plexiglas,available from Rohn & Haas Co.); and polyalkylenes such as polyethyleneand polypropylene. The polyalkylenes are presently preferred for theadsorbent layer because of their low working temperatures, inertness tobromine, and availability at low cost.

The anode coating may be simply electrically nonconductive particles asdescribed pressed into and bonded to the preformed electroconductivemember.

A thicker layer, having a thickness on the order of 7 that of thesurface layer of the cathode is also useful.

For forming such a layer, a polyalkylene bonding agent is preferred, andthe mixture of polyalkylene is bonded to the already pressed and bondedparticle coating, in the same manner as hereinafter described foradhering the adsorbent layer to a base electroconductive layer withreference to FIGS. 80, 8b, 9a, and 9b.

In addition to the illustrated poly (vinylidene fluoride) bonding agentfor the electroconductive layers of the composite electrodes 22, and theillustrated polyethylene bonding agent in the various adsorbentlayers,,other bonding agents or mixtures of bonding agentsmay besubstituted therefor.

Materials suitable for forming such layers are set forth in theapplicants copending U.S. patent applications: Ser. No. 867,799 filedOct. 20, 1969, now U.S. Pat. No. 3,640,770 entitled BATTERY; and, Ser.No. 872,993, filed Oct. 31, 1969 now U.S. Pat. No. 3,642,538, entitledMETAL HALIDE BATTERY to which reference is made for details of selectionof bonding agents. Another useful bonding agent system is the mixture ofa polyfluorocarbon with a minor amount of a polyethylene orpolypropylene, such as disclosed in the assignees copending U.S. patentapplication, entitled Electroconductive Composition", Ser. No. 109,156,filed Jan. 25, 1971, in the names of Ralph Zito, J r., and Edward M.Russell. Where such a mixture is used, bonding of thepolyfluorocarbon-bonded electroconductive layers to thepolyethylene-bonded activated carbon layer is simpler. For example, itis possible, but not necessarily preferable, to eliminate the step ofadhering a thin activated carbon layer, as shown in FIGS. 8a, 8b, to theelectroconductive layer prior to forming the bromine-adsorbent device.

Where the electroconductive layer is to be part of a terminal anode orterminal cathode having a brominecorrodible screen, the bonding agentmay be any of those previously set forth for the composite electrodes,except the bromine-permeable polyethylene .and polypropylene. Wherebromine-degradable polymers, such as vinyl chloride and vinylidenechloride are employed, in electroconductive layers, whether in compositeor terminal electrodes, the maximum bromine concentration at theelectroconductive layers should not exceed about 0.5 M.

Whatever the composition of the various electrodes, each electrodeshould have a total interface resistance, per square inch of thecross-sectional electrolyte-contacting surface area, not greater thanabout 0.05 ohms. In addition, each electroconductive layer 28, 29, 46,58, 59 should have a volume resistivity, p, such that pd is not greaterthan about 0.1 ohm-in, where d is the thickness of the electroconductivelayer.

For terminal electrodes, the electroconductive layer which is notexposed to the remainder of the battery (the layer 28, for example, ofterminal cathode 14) need not contain any electroconductive carbonparticles, but may be simply a sheet of the identical bonding agentsused in the electroconductive layer to which it is secured through thecopper screen.

FIGS. 7-10 show a method for making the electrodes 14, 18, 22, themethod for forming the composite electrode 22 being used asa particularillustration. The electroconductive layer 46 shown in FIGS. 7a and 7b isformed by mixing together 50 grams of poly (vinylidene fluoride)(Pennwalt "Kynar 301 with 50 grams of highly electroconductive graphiteparticles (Dixon No. 1 1 12). This mixture is introduced into a 10 X 12inch frame and trowelled until level. The frame is placed in a moldformed of two platens, which are then heated to 450 for 1 minute withoutpressure, and then are pressed at 500 psi and 450F for 3 more minutes.The frame is then transferred to platens at room temperature and cooledbetween those platens under 500 psi for 2 more minutes. The resultantlayer is about 25 mils thick. Where a terminal anode 18 or a terminalcathode 14 is to be made, two such layers 46 are provided. A copperscreen is placed between the two electroconductive layers, with aportion of the screen protruding for making ultimate electricalconnection thereto, and the layers and screen are placed betweenplatens, and heated for 1 minute without pressure at 450F and for 2 moreminutes at 500 psi and 450F, transferred to platens at room temperature,and cooled for 2 minutes between these platens at 500 psi.

Referring to FIGS. 8a and 8b, there is spread in the bottom of a frameas described above a thin layer (about 10 mils) of Celite 560, a fluxcalcined diatomaceous earth available from Johns Manville Company whichhas a dry density of 19.5 pounds per cubic foot, a particle size suchthat all of the particles pass a mesh screen and 60 percent of theparticles by weight are retained by a mesh screen and a water absorptionof about 220 percent by weight. These particles have pores witheffective diameters less than 1 micron. The electroconductive layer 46is placed in the frame on top of the Celite layer, and there is spreadonto the other exposed major surface of the electroconductive layer 46 athin layer (about 2 grams), confined within a frame enclosing a 9 X 9inch area, of activated carbon particles (Barneby Cheney UU Grade). The9 X 9 inch frame is removed, and the Celite-electroconductivelayer-activated carbon is placed between two platens, heated to 375F forone minute without pressure, compressed between the platens at 375F and400 psi for 2 minutes, transferred to platens at room temperature andcooled for 2 minutes between these platens at 400 psi. Both surfaces ofthe resultant sheet are then scrubbed with a conventional stiffscrubbing brush to remove non-bonded activated carbon and Celiteparticles. The 375 temperature having been sufficient to soften slightlythe poly (vinylidene fluoride) binder of the electroconductive layerwithout affecting the basic shape and form of the layer, the Celite andactivated carbon particles are bonded to opposite sides of the layer bythe poly (vinylidene fluoride) bonding agent, forming the integralelectrode structure shown in FIGS. 8a and 8b.

Onto the activated carbon surface of this electroconductive structure istrowelled, within the 9 X 9 inch frame, a mixture consisting of 72 gramsby weight of the aforesaid activated carbon and about 3.6 grams byweight of polyethylene particles (FN-510 powder, available from U.S.Indus. Chem. Co.), this mixture having been previously ball milled for14 minutes. The mixture is trowelled in such a manner as to leaverounded surfaces around the edges of the activated carbon layer, so thatthe top surface has an area of about 8% X 34 inches, with thecross-sectional area of the activated carbon gradually increases toabout 9 X 9 inches about half way down the total thickness of the layer(about 60 mils down from the top surface). This electrode structure isshown in FIGS. 9a and 9b.

There is trowelled onto this activated carbonpolyethylene mixture withinthe 9 X 9 inch frame a mixture consisting of 32.4 grams of the aforesaidCelite 560 and 3.6 grams of the aforesaid polyethylene. The Celitemixture is trowelled in such a manner as to fill the edge portions ofthe activated carbon layer of FIGS. 9a, 9b, to provide, as shown inFIGS. a, 10b, a com posite activated carbon Celite layer of rectangularcross-section. The entire composite of FIGS. 10a, 10b is placed betweenplatens, heated to 300F for 7 minutes, pressed at 75 psi and 300F for 7minutes, transferred to platens at room temperature, and cooled at 75psi for 7 minutes. The resulting surface (Celite) layer retainsessentially the porocity of the individual particles. The interiorportion (exclusive of the edges) of the activated carbon is aboul milsthick, and the interior portion (exclusive of the edges) of the Celitelayer is about mils thick.

Other embodiments will occur to one skilled in the art and are withinthe following claims.

What is claimed is:

l. A rechargeable metal halide battery, in which a salt of anelectroplatable metal and a halogen selected from the class consistingof chlorine, bromine and iodine is electrolyzed from solution in aliquid electrolyte medium during the charging cycle and reformed duringthe discharging cycle, an electrode having a cathode structurecomprising a halogen-inert, carbon containing electroconductive layer,substantially impermeable to said liquid medium and substantiallyimpermeable and inert to halogen between discharging cycles, andliquid-permeable halogen entrapment structure bonded to saidelectroconductive layer along one major surface of saidelectroconductive layer, said entrapment structure being permeable tosaid electrolyte, inert to said halogen, and comprising a halogenadsorbent layer adjacent said electroconductive layer, and, at leastalong and bonded to the major surface of said adsorbent layer, oppositethe 5. The battery of claim 4 wherein a major portion by weight of saidelectrically non-conductive particles are retained by a 150 mesh screen.

6. The battery of claim 1 wherein the average effective diameter of thepores of said particles is less than about 1 micron.

7. The battery of claim 1 wherein said halogen is bromine, and saidadsorbent layer comprises bromineadsorbent activated carbon particlesbonded together into an integral adsorbent mass by a bromine-inertbonding agent.

8. The battery of claim 7 wherein said surface layer is of greaterthickness at its edges than in the interior portion thereof and has asubstantially planar exposed major surface, and the edges of saidadsorbent layer are of reduced thickness sized to accommodate saidgreater thickness of the edges of said surface layer.

9. The battery of claim 8 wherein substantially all of said particlespass through a 100 mesh screen, and the surface of said adsorbent layerwhich is bonded to said electroconductive layer, a relatively thinsurface layer formed of solid, porous, electrically non-conductive,halogen-inert, electrolyte-inert, and halogen-nonadsorbent particles,having an average largest dimension less than about 10 mils, and ahalogen-inert bonding agent bonding said particles together into anintegral electrically nonconductive mass but with said particlessufficiently exposed so that said surface layer essentially retains theporosity of said particles, portions of said electroconductive layer andsaid adsorbent layer which would otherwise be exposed being coated witha substantially gas-impermeable sealant.

2. The battery of claim 1 wherein said surface layer comprises at leastabout 90 percent by weight of said electrically non-conductiveparticles.

3. The battery of claim 1 wherein said electrically non-conductiveparticles consist essentially of diatomaceous earth.

4. The battery of claim 1 wherein substantially all of said particlespass through a 100 screen.

said portion of said surface layer has an average thickness of at leastabout 10 mils.

10. The battery of claim 1 wherein the major surface of saidelectroconductive layer opposite to the major surface bonded to saidentrapment structure is arranged for exposure to said liquid medium toprovide an extended electroplating surface for said metal.

11. The battery of claim 10 wherein said opposite major surface of saidelectroconductive layer includes a coating of said electricallynon-conductive particles firmly bonded to said surface.

12. A rechargeable metal halide battery, in which a salt of anelectroplatable metal and a halogen selected from the class consistingof chlorine, bromine and iodine is electrolyzed from solution in aliquid electrolyte medium during the charging cycle and reformed duringthe discharging cycle, an electrode structure comprising ahalogen-inert, carbon containing electroconductive member substantiallyimpermeable to said liquid medium, and substantially impermeable tohalogen between discharging cycles,

said member having an exposed surface adapted to be arranged as anelectroplating surface for said metal, and a coating consistingessentially of discrete, solid, halogen-inert, electrolyte-inert,halogen non-adsorbent, porous, electrically nonconductive particles onsaid exposed surface, said particles having an average largest diameterless than about 10 mils, said coating being firmly bonded to saidsurface, with said particles substantially exposed on said surface sothat said coating essentially retains the porosity of said particles,said coating being of a thickness sufficient to substantially cover saidsurface.

13. The battery of claim 12 wherein said coating has an averagethickness of at least about 5 mils.

14. The battery of claim 12 wherein said electroconductive membercomprises 25 to percent by weight of highly electroconductive carbonparticles, and the remainder by weight of a bonding agent substantiallyinert to said halogen and to said liquid medium, bonding said carbonparticles together into an integral electroconductive mass.

15. The battery of claim 14 wherein said electrically non-conductiveparticles are bonded to said electroconductive member by said bondingagent.

* II! l

1. A rechargeable metal halide battery, in which a salt of anelectroplatable metal and a halogen selected from the class consistingof chlorine, bromine and iodine is electrolyzed from solution in aliquid electrolyte medium during the charging cycle and reformed duringthe discharging cycle, an electrode having a cathode structurecomprising a halogen-inert, carbon containing electroconductive layer,substantially impermeable to said liquid medium and substantiallyimpermeable and inert to halogen between discharging cycles, and aliquid-permeable halogen entrapment structure bonded to saidelectroconductive layer along one major surface of saidelectroconductive layer, said entrapment structure being permeable tosaid electrolyte, inert to said halogen, and comprising a halogenadsorbent layer adjacent said electroconductive layer, and, at leastalong and bonded to the major surface of said adsorbent layer, oppositethe surface of said adsorbent layer which is bonded to saidelectroconductive layer, a relatively thin surface layer formed ofsolid, porous, electrically non-conductive, halogen-inert,electrolyte-inert, and halogen-nonadsorbent particles, having an averagelargest dImension less than about 10 mils, and a halogen-inert bondingagent bonding said particles together into an integral electricallynon-conductive mass but with said particles sufficiently exposed so thatsaid surface layer essentially retains the porosity of said particles,portions of said electroconductive layer and said adsorbent layer whichwould otherwise be exposed being coated with a substantiallygas-impermeable sealant.
 2. The battery of claim 1 wherein said surfacelayer comprises at least about 90 percent by weight of said electricallynon-conductive particles.
 3. The battery of claim 1 wherein saidelectrically non-conductive particles consist essentially ofdiatomaceous earth.
 4. The battery of claim 1 wherein substantially allof said particles pass through a 100 screen.
 5. The battery of claim 4wherein a major portion by weight of said electrically non-conductiveparticles are retained by a 150 mesh screen.
 6. The battery of claim 1wherein the average effective diameter of the pores of said particles isless than about 1 micron.
 7. The battery of claim 1 wherein said halogenis bromine, and said adsorbent layer comprises bromine-adsorbentactivated carbon particles bonded together into an integral adsorbentmass by a bromine-inert bonding agent.
 8. The battery of claim 7 whereinsaid surface layer is of greater thickness at its edges than in theinterior portion thereof and has a substantially planar exposed majorsurface, and the edges of said adsorbent layer are of reduced thicknesssized to accommodate said greater thickness of the edges of said surfacelayer.
 9. The battery of claim 8 wherein substantially all of saidparticles pass through a 100 mesh screen, and the said portion of saidsurface layer has an average thickness of at least about 10 mils. 10.The battery of claim 1 wherein the major surface of saidelectroconductive layer opposite to the major surface bonded to saidentrapment structure is arranged for exposure to said liquid medium toprovide an extended electroplating surface for said metal.
 11. Thebattery of claim 10 wherein said opposite major surface of saidelectroconductive layer includes a coating of said electricallynon-conductive particles firmly bonded to said surface.
 12. Arechargeable metal halide battery, in which a salt of an electroplatablemetal and a halogen selected from the class consisting of chlorine,bromine and iodine is electrolyzed from solution in a liquid electrolytemedium during the charging cycle and reformed during the dischargingcycle, an electrode structure comprising a halogen-inert, carboncontaining electroconductive member substantially impermeable to saidliquid medium, and substantially impermeable to halogen betweendischarging cycles, said member having an exposed surface adapted to bearranged as an electroplating surface for said metal, and a coatingconsisting essentially of discrete, solid, halogen-inert,electrolyte-inert, halogen non-adsorbent, porous, electricallynon-conductive particles on said exposed surface, said particles havingan average largest diameter less than about 10 mils, said coating beingfirmly bonded to said surface, with said particles substantially exposedon said surface so that said coating essentially retains the porosity ofsaid particles, said coating being of a thickness sufficient tosubstantially cover said surface.
 13. The battery of claim 12 whereinsaid coating has an average thickness of at least about 5 mils.
 14. Thebattery of claim 12 wherein said electroconductive member comprises 25to 75 percent by weight of highly electroconductive carbon particles,and the remainder by weight of a bonding agent substantially inert tosaid halogen and to said liquid medium, bonding said carbon particlestogether into an integral electroconductive mass.